You share----as in seed after you finish downloading, or let people connect to you in general. If you download until finished, and then close Azureus, then you are not sharing. Hit is called a hit-n-run, and no one likes people who do this.
Next time you finish downloading a good file, show your appreciation by leaving your computer and Azureus on overnight. You can seed while you sleep, and the next morning your share ratio should improve a good amount. Once you upload as much as you have downloaded, your ratio will be 1:1, which is where everyone should be (at the very least).